A conventionally known projection apparatus projects light emitted from a light irradiation device (e.g., a projector) onto a reflector (e.g., a screen). Reflection of the light projected onto the screen enables an observer to perceive an image on the screen. However, the screen used in such projection apparatus is usually opaque. Such opaque screen prevents the observer from perceiving (viewing) the sight behind the screen through the screen from inside the projection apparatus (i.e., from the light irradiation device side).
In recent years, conventional projection apparatuses have been modified to thus develop projection apparatuses (head-up displays) that each use a reflector having light transmissivity (light transmissive reflector). For example, Patent Document 1 discloses an apparatus for displaying an image by reflecting light emitted from a light irradiation device toward the windshield of an automobile that serves as a light transmissive reflector.
A light transmissive reflector not only reflects light emitted from a light irradiation device, but also passes light incident from behind the light transmissive reflector. Thus, an observer not only perceives the image projected onto the light transmissive reflector, but can also view the sight behind the light transmissive reflector therethrough. This enables the driver of an automobile to see driving information, such as location coordinates and a speedometer indication, displayed on the windshield that serves as a light transmissive reflector, and at the same time, to view the sight outside the automobile through the windshield. Thus, the driver can obtain driving information during driving the automobile without moving his or her eyes away from the traveling direction.
When a head-up display is applied to a vehicle (e.g., an automobile) as described above, the density of an image displayed on the light transmissive reflector is particularly important for safe driving of the vehicle.
More specifically, too high a density of the image displayed on the light transmissive reflector presents certain difficulties in viewing the sight outside the vehicle through the light transmissive reflector, and may thus prevent safe driving of the vehicle. Conversely, too low a density of the image displayed on the light transmissive reflector prevents the driver from obtaining sufficient driving information. In this case, the driver tends to pay more attention on the displayed image to obtain sufficient driving information. This, in turn, distracts attention of the driver from the sight outside the vehicle, and may thus prevent safe driving of the vehicle.
Thus, if a head-up display is applied to a vehicle, the density of an image projected needs to be low enough to allow the driver to view the sight outside the vehicle through the light transmissive reflector to a sufficient degree, and also high enough to allow the driver to quickly read the driving information.
[Patent Document 1] JP 2014-85657 A